In this Finnish-based study, professional composers ( n = 2), student music teachers ( n = 6), eighth-grade comprehensive school pupils studying in an extended music program ( n = 22), and university and school music educators ( n = 2) were brought together to pilot a partnership model for teaching and learning music composition. The one-year project offered (a) a composition pedagogy course for student music teachers, including lectures by composers and a seminar and teacher training in a comprehensive school, and (b) collaborative composition workshops for eighth-grade pupils under the supervision of student music teachers, professional composers, and a school music teacher. In Part 1, we conducted a survey to investigate pupils’ experiences relating to their creative musical agencies and basic psychological needs. In Part 2, we used a phenomenographic approach to examine educators’ perceptions of student music teachers’ development as composition teachers. In Part 3, we used phenomenography to study the student music teachers’ experiences of their development as composition teachers. The results indicate that (a) the musical agency and basic psychological needs of the eighth-grade pupils were supported; (b) the educators succeeded in planning and implementing the pilot course, taking into account both the social and pedagogical aspects of collaborative composition; and (c) the student music teachers felt that their attitudes toward teaching music composition changed and that their confidence increased during the project. We recommend future modifications to the present educational model and consider its potential implications for music education in the Finnish context.